Advances in recombinant DNA technology coupled with advances in plant transformation and regeneration technology have made it possible to introduce new genetic material into plant cells, plants or plant tissue, thus introducing new traits, e.g., phenotypes, that enhance the value of the plant or plant tissue. Recent demonstrations of genetically engineered plants resistant to pathogens (EP-A 240 332 and EP-A 223 452) or insects (Vaeck, M. et al., Nature 328: 33 (1987)) and the production of herbicide tolerant plants (DeBlock, M. et al., EMBO J. 6: 2513 (1987)) highlight the potential for crop improvement. The target crops can range from trees and shrubs to ornamental flowers and field crops. Indeed, it is clear that the "crop" can also be a culture of plant tissue grown in a bioreactor as a source for some natural product.
A. General Overview of Plant Transformation Technology
Various methods are known in the art to accomplish the genetic transformation of plants and plant tissues (i.e., the stable introduction of foreign DNA into plants). These include transformation by Agrobacterium species and transformation by direct gene transfer.
1. Agrobacterium-mediated Transformations
A. tumefaciens is the etiologic agent of crown gall, a disease of a wide range of dicotyledons and gymnosperms, that results in the formation of tumors or galls in plant tissue at the site of infection. Agrobacterium, which normally infects the plant at wound sites, carries a large extrachromosomal element called the Ti (tumor-inducing) plasmid.
Ti plasmids contain two regions required for tumorigenicity. One region is the T-DNA (transferred-DNA) which is the DNA sequence that is ultimately found stably transferred to plant genomic DNA. The other region required for tumorigenicity is the vir (virulence) region which has been implicated in the transfer mechanism. Although the vir region is absolutely required for stable transformation, the vir DNA is not actually transferred to the infected plant. Transformation of plant cells mediated by infection with Agrobacterium tumefaciens and subsequent transfer of the T-DNA alone have been well documented. See, for example, Bevan, M. W. and Chilton, M-D., Int. Rev. Genet. 16: 357 (1982).
After several years of intense research in many laboratories, the Agrobacterium system has been developed to permit routine transformation of a variety of plant tissue. Representative species transformed in this manner include tobacco, tomato, sunflower, cotton, rapeseed, potato, soybean, and poplar. While the host range for Ti plasmid transformation using A. tumefaciens as the infecting agent is known to be very large, tobacco has been a host of choice in laboratory experiments because of its ease of manipulation.
Agrobacterium rhizogenes has also been used as a vector for plant transformation. This bacterium, which incites hairy root formation in many dicotyledonous plant species, carries a large extrachromosomal element called an Ri (root-inducing) plasmid which functions in a manner analogous to the Ti plasmid of A. tumefaciens. Transformation using A. rhizogenes has developed analogously to that of A. tumefaciens and has been successfully utilized to transform, for example, alfalfa, Solanum nigrumL., and poplar.
2. Direct Gene Transfer
Several so-called direct gene transfer procedures have been developed to transform plants and plant tissues without the use of an Agrobacterium intermediate (see, for example, Koziel et al., Biotechnology 11: 194-200 (1993); U.S. application Ser. No. 08/008,374, filed Jan. 25, 1993, herein incorporated by reference in its entirety). In the direct transformation of protoplasts the uptake of exogenous genetic material into a protoplast may be enhanced by use of a chemical agent or electric field. The exogenous material may then be integrated into the nuclear genome. The early work was conducted in the dicot tobacco where it was shown that the foreign DNA was incorporated and transmitted to progeny plants, see e.g. Paszkowski, J. et al., EMBO J. 3: 2717 (1984); and Potrykus, I. et al., Mol. Gen. Genet. 199: 169 (1985).
Monocot protoplasts have also been transformed by this procedure in, for example, Triticum monococcum, Lolium multiflorum (Italian ryegrass), maize, and Black Mexican sweet corn.
Alternatively exogenous DNA can be introduced into cells or protoplasts by microinjection. A solution of plasmid DNA is injected directly into the cell with a finely pulled glass needle. In this manner alfalfa protoplasts have been transformed by a variety of plasmids, see e.g. Reich, T. J. et al., Bio/Technology 4: 1001 (1986).
A more recently developed procedure for direct gene transfer involves bombardment of cells by microprojectiles carrying DNA, see Klein, T. M. et al., Nature 327: 70 (1987). In this procedure tungsten particles coated with the exogenous DNA are accelerated toward the target cells, resulting in at least transient expression in the example reported (onion).
B. Regeneration of Transformed Plant Tissue
Just as there is a variety of methods for the transformation of plant tissue, there is a variety of methods for the regeneration of plants from plant tissue. The particular method of regeneration will depend on the starting plant tissue and the particular plant species to be regenerated. In recent years it has become possible to regenerate many species of plants from callus tissue derived from plant explants. The plants which can be regenerated from callus include monocots, such as corn, rice, barley, wheat and rye, and dicots, such as sunflower, soybean, cotton, rapeseed and tobacco.
Regeneration of plants from tissue transformed with A. tumefaciens has been demonstrated for several species of plants. These include sunflower, tomato, white clover, rapeseed, cotton, tobacco, and poplar. The regeneration of alfalfa from tissue transformed with A. rhizogenes has also been demonstrated. Plant regeneration from protoplasts is a particularly useful technique, see Evans, D. A. et al., in: "Handbook of Plant Cell Culture", Vol. 1, MacMillan Publ. Co., 1983, p. 124. When a plant species can be regenerated from protoplasts, then direct gene transfer procedures can be utilized, and transformation is not dependent on the use of A. tumefaciens. Regeneration of plants from protoplasts has been demonstrated for rice, tobacco, rapeseed, potato, eggplant, cucumber, poplar, and corn.
Various plant tissues may be utilized for transformation with foreign DNA. For instance, cotyledon shoot cultures of tomato have been utilized for Agrobacterium mediated transformation and regeneration of plants (see European application 0249432). Further examples include Brassica species (see WO 87/07299) and woody plant species, particularly poplar (see U.S. Pat. No. 4,795,855, incorporated by reference herein in its entirety).
The technological advances in plant transformation and regeneration technology highlight the potential for crop improvement via genetic engineering. There have been reports of genetically engineered tobacco and tomato plants which are resistant to infections of, for example, tobacco mosaic virus (TMV) and resistant to different classes of herbicides. Insect resistance has been engineered in tobacco and tomato plants.
C. Cell Cultures
The potential for genetic engineering is not limited to field crops but includes improvements in ornamentals, forage crops and trees. A less obvious goal for plant biotechnology, which includes both genetic engineering and tissue culture applications, is the enhanced production of a vast array of plant-derived chemical compounds including flavors, fragrances, pigments, natural sweeteners, industrial feedstocks, antimicrobials and pharmaceuticals. In most instances these compounds belong to a rather broad metabolic group, collectively denoted as secondary products. Plants may produce such secondary products to ward off potential predators, attract pollinators, or combat infectious diseases.
Plant cell cultures can be established from an impressive array of plant species and may be propagated in a bioreactor. Typical plant species include most of those that produce secondary products of commercial interest. It has been clearly demonstrated in a number of agriculturally important crop plants that stable genetic variants arising from the tissue culture of plant somatic cells (somaclonal variation) can be induced and selected. Numerous advantages flow from plant tissue culture production of secondary compounds. These include (1) the possibility of increased purity of the resultant product, (2) the conversion of inexpensive precursors into expensive end products by biotransformation, and (3) the potential for feeding substrate analogs to the culture to create novel compounds.
D. Advantages of Controlled Gene Expression
Whether the target of genetic engineering of plants is a field crop, ornamental shrub, flower, tree or a tissue culture for use in a bioreactor, a principal advantage to be realized is the control of expression of the chimeric gene so that it is expressed only at the appropriate time and to the appropriate extent, and in some situations in particular pans of the plant. For example, in order to achieve a desirable phenotype the chimeric gene may need to be expressed at levels of 1% of the total protein or higher. This may well be the case for fungal resistance due to chimeric chitinase expression or insect resistance due to increased proteinase inhibitor expression. In these cases the energy expended to produce high levels of the foreign protein may result in a detriment to the plant whereas, if the gene were expressed only when desired, for instance when a fungal or insect infestation is imminent, the drain on energy, and therefore yield, could be reduced.
Alternatively, the phenotype expressed by the chimeric gene could result in adverse effects to the plant if expressed at inappropriate times during development. For example, if the chimeric gene product were a plant hormone that induced pod abscission, early expression could bring about abscission of the fruit from the plant before the seed had matured, resulting in decreased yield. In this case it would be much more advantageous to induce the expression of this type of gene to a time when pod abscission is preferred, or least injurious to the plant.
For tissue in culture or in a bioreactor the untimely production of a secondary product could lead to a decrease in the growth rate of the culture resulting in a decrease in the yield of the product. Therefore, it would be advantageous to allow the culture to grow without expressing the secondary product and then induce the chimeric gene at an appropriate time to allow for an optimized expression of the desired product.
In view of considerations like these, as well as others, it is clear that control of the time, extent and/or site of expression of the chimeric gene in plants or plant tissues would be highly desirable. Control that could be exercised easily in a field, a greenhouse or a bioreactor would be of particular commercial value.
E. Known Regulatable Gene Expression Systems in Plants
Several plant genes are known to be induced by various internal and external factors including plant hormones, heat shock, chemicals, pathogens, lack of oxygen and light. While few of these systems have been described in detail, in the best characterized, an increased accumulation of mRNA leads to an increased level of specific protein product.
As an example of gene regulation by a plant hormone, abscissic acid (ABA) has been shown to induce the late embryogenesis abundant mRNAs of cotton, see Galau, G. A. et al., Plant Mol. Biol. 7: 155 (1986). In another example, gibberellic acid (GA3) induces malate synthase transcripts in castor bean seeds and alpha-amylase isozymes in barley aleurone layers, see Rodriguez, D. et al., Plant Mol. Biol. 9: 227 (1987); Nolan, R. C. et al., Plant Mol. Biol. 8: 13 (1987).
The regulation of heat shock protein genes of soybean has been studied in detail. Treatment of plants for several hours at 40.degree. C. results in the de novo synthesis of several so-called heat shock proteins (Key, J. et al., Proc. Natl. Acad. Sci. USA, 78: 3526 (1981)). Several such genes have been isolated and their regulation studied in detail. The expression of these genes is primarily controlled at the level of transcription. The promoter of the hsp70 gene has been fused to the neomycin phosphotransferase II (NptlI) gene and the chimeric gene has been shown to be induced by heat shock (Spena, A. et al., EMBO J. 4: 2736 (1985)) albeit at a lower level than the endogenous heat shock genes.
Another class of inducible genes in plants include the light regulated genes, most notably the nuclear encoded gene for the small subunit of ribulose 1,5-bisphosphate carboxylase (RUBISCO). Morelli, G. et al., Nature 315: 200 (1985)) and Hererra-Estrella, L. et al., Nature 310: 115 (1984)) have demonstrated that the 5' flanking sequences of a pea RUBISCO gene can confer light inducibility to a reporter gene when attached in a chimeric fashion. This observation has been extended to other light inducible genes such as the chlorophyll a/b binding protein.
The alcohol dehydrogenase (adh) genes of maize have been extensively studied. The adh1-s gene from maize was isolated and a portion of the 5' flanking DNA was shown to be capable of inducing the expression of a chimeric reporter gene (e.g., chloramphenicol acetyl transferase, CAT) when the transiently transformed tissue was subjected to anaerobic conditions (Howard, E. et al., Planta 170: 535 (1987)).
A group of chemicals known as safeners have been developed to protect or "safen" crops against potentially injurious applications of herbicides. While a general mechanism for the action of such compounds has not been fully developed, regulation of naturally regulatable genes by such compounds is one possible mechanism. It has recently been reported that higher levels of a glutathione-S-transferase (GST) are induced in maize treated with the safener 2-chloro-4-(trifluoromethyl)-5-methyl-thiazolecarboxylic acid benzyl ester, see Wiegand, R. C. et al., Plant Mol. Biol. 7: 235 (1986). Although the level of GST mRNA is elevated upon treatment with the safener, the mechanism leading to the elevation was not reported.
Many plants, when reacting hypersensitively toward various pathogens, are stimulated to produce a group of acid-extractable, low molecular weight pathogenesis-related (PR) proteins (Van Loon, L. C., Plant Mol. Biol. 4: 111 (1985)). Of particular interest, however, is the observation that these same PR proteins accumulate to high levels in plants treated with chemicals such as polyacrylic acid and acetylsalicylic acid (Gianinazzi, S. et al., J. Gen. Virol. 23:1 (1974); White, R. F., Virology 99: 410 (1979)). The presence of PR proteins has been correlated with the induction of both a local and systemic resistance against a broad range of pathogens. An interspecific tobacco hybrid resistant to tobacco mosaic virus (TMV) was shown to express the PR-proteins constitutively (Ahl, P. et al., Plant Sci. Lett. 26: 173 (1982)). Furthermore, immunoprecipitation of in vitro translation products using mRNA from either TMV-infected or chemically treated tobacco (Cornelissen, B. J. C. et al., EMBO J. 5: 37 (1986); Carr, J. P. et al., Proc. Natl. Acad. Sci. USA 82: 7999 (1985)) indicated that the increased level of PR-protein was a result of RNA accumulation. Therefore, induction of PR protein genes by chemicals or pathogens provides a method to address the problem of chemically regulating gene expression in plants and plant tissue.
F. Chemical Regulation of Expression
In some cases it will be desirable to control the time and/or extent of the expression of introduced genetic material in plants, plant cells or plant tissue. An ideal situation would be the regulation of expression of an engineered trait at will via a regulating intermediate that could be easily applied to field crops, ornamental shrubs, bioreactors, etc. This situation can now be realized by the present invention which is directed to, among other things, a chemically regulatable chimeric gene expression system comprising a chemically regulatable, non-coding DNA sequence coupled, for example, to a gene encoding a phenotypic trait, such that the expression of that trait is under the control of the regulator, e.g. such that expression from the regulated gene is determined by the presence or absence of a chemical regulator. This system is the first demonstration of the concept of chemical regulation of chimeric gene expression in plants or plant tissue. As such it enables the production of transgenic plants or plant tissue and the control of traits expressed as a function of a chemical regulator.
The present invention also teaches the external manipulation of the expression of endogenous genes which contain chemically regulatable sequences by the application of a chemical regulator (see Ward, E. et al., Plant Cell 3: 1085-1094 (1991); Williams et al., Bio/Technology 10: 540-543 (1992); and Uknes, S. et al., Plant Cell 5: 159-169 (1993). The control provided is somewhat limited, however, due to the responsiveness of such sequences to endogenous chemical metabolites and cell signals as well as externally applied chemical regulators. In yet another aspect of the invention, alterations are taught which block the responsiveness of these genes to endogenous signals while maintaining responsiveness to externally applied chemical regulators.
G. Insect Resistance
Pest infestation of crop plants causes considerable loss of yield throughout the world and most crops grown in the U.S. suffer infestation, particularly from insects. Major insect pests in the U.S. include the European Corn Borer (Ostrinia nubilalis) in maize, the cotton bollworm Heliothis zea) and the pink bollworm (Pectinophora gossypiella) in cotton and the tobacco budworm (Hellothis virescens) in tobacco. Resistance to pests is difficult to achieve using conventional breeding programs and typically pests have been controlled using chemical pesticides.
Recent advances in molecular biology and plant transformation technology have demonstrated the possibility of expressing in transgenic plants genes encoding insecticidal proteins; this represents a novel approach in the production of crop plants resistant to pests. Most notably, the expression of genes encoding the Bacillus thuringiensis .delta.-endotoxin has been successful in a wide range of plant species, and the analysis of transgenic lines expressing such genes has been well documented (Vaeck et al., Nature 328: 33-37 (1987); Fischoff et at., Biotechnology 5: 807-813 (1987); Carozzi et al., Plant Mol. Biol. 20: 539-548 (1992); Koziel et al., Biotechnology 11: 194-200 (1993)). Other insecticidal genes have been used successfully in generating insect resistant transgenic plants.
One approach has been the overexpression of genes encoding insect enzyme inhibitors such as trypsin inhibitors or seed proteins with known insecticidal activity, such as lectins (Hilder et al., Nature 330: 160-163 (1987)). Indeed, plants expressing both the cowpea trypsin inhibitor and pea lectin were shown to have additive effects in providing insect resistance (Boulter et al., Crop Protection 9: 351-354 (1990)). In cases where pests are able infest parts of the plant or tissues not readily accessible to conventional pesticides, a transgenic approach may be more successful than the use of conventional pesticides.
For example, the tobacco budworm Heliothis is well known to be difficult to control using pesticides because it burrows deep into the plant tissue. Additionally, some pests of roots, such as nematodes, are not readily controlled by foliar applications of pesticides. An advantage in the use of transgenic plants expressing insecticidal proteins is the controlled expression of the proteins in all desired tissues.
Chitinases catalyze the hydrolysis of chitin, a .beta.-1,4-linked homopolymer of N-acetyl-D-glucosamine. Several different plant chitinases have been described and the cDNA sequences for some of these have been reported (Meins et al., Mol. Gen. Genet. 232: 460-469 (1992)). Based on structural characteristics, three classes have been distinguished. Class I chitinases have two structural domains, a cysteine-rich amino-terminal hevein domain and a carboxyterminal catalytic domain separated from the former by a variable spacer. Class II chitinases lack the cysteine-rich hevein domain and all or part of the variable spacer, but retain the catalytic domain. Class III chitinases lack the hevein domain and contain a catalytic domain that shares no significant homology with that of the class I or class II enzymes.
Class I chitinase gene expression is induced by ethylene, whereas class II and class III chitinase gene expression is induced in the SAR response. The chitinase/lysozyme disclosed in U.S. application Ser. No. 07/329,018 and the chitinase/lysozyme disclosed in U.S. application Ser. No. 07/580,431 (provided herein as SEQ ID Nos. 29 and 30, respectively) are class III chitinases. It is well known that the level of chitinase activity of plants increases dramatically after pathogen invasion (Mauch et al., Plant Physiol. 76: 607-611 (1984)) and this is presumably due to the host planes attempts to degrade the chitin of the fungal cell wall. Furthermore, chitinase has been shown in vitro to inhibit fungal and insect growth, and in transgenic plants a bacterial chitinase has been shown to exhibit inhibitory effects towards numerous pathogens and pests including insects (Suslow & Jones WPO 90-231246; U.S. Pat. Nos. 4,940,840 and 4,751,081; herein incorporated by reference in their entirety).
H. Resistance Response of Plants to Pathogen Infection
For over 90 years, scientists and naturalists have observed that when plants survive pathogen infection they develop an increased resistance to subsequent infections. In 1933, a phenomenon termed "physiological acquired immunity" was described in an extensive literature review by Chester, K. S., Quart. Rev. Biol. 8: 275-324 (1933). At that time, scientists believed they were investigating a phenomenon analogous to the immune response in mammals. In retrospect, at least three different processes were being called "aquired immunity": viral cross protection, antagonism (or biocontrol), and what we now refer to as systemic acquired resistance (SAR).
1. Systemic Acquired Resistance (SAR)
The first systematic study of SAR was published by A. Frank Ross in 1961. Using tobacco mosaic virus (TMV) on local lesion hosts, Ross demonstrated that infections of TMV were restricted by a prior infection. This resistance was effective against not only TMV, but also tobacco necrosis virus and certain bacterial pathogens. Ross coined the term "systemic acquired resistance" to refer to the inducible systemic resistance (Ross, A. F., Virology 14: 340-358 (1961)) and "localized acquired resistance" (LAR) to describe the resistance induced in inoculated leaves (Ross, A. F., Virology 14: 329-339 (1961)). It is still unclear whether SAR and LAR are aspects of the same response or distinct processes.
In the past 30 years, SAR has been demonstrated in many plant species and the spectrum of resistance has been broadened to include not only viruses and bacteria, but also many agronomically important phytopathogenic fungi (see Kuc, J., BioScience 32: 854-860 (1982). However, understanding of the biochemical events leading to the establishment of SAR had not progressed substantially until the past dozen years. In 1982, the accumulation of a group of extracellular proteins called pathogenesis-related (PR) proteins were reported to correlate with the onset of SAR (Van Loon, L. C. et al., Neth. J. Plant. Path. 88: 237-256 (1982)). In 1979, salicylic acid (SA) and certain benzoic acid derivatives were reported to be able to induce both resistance and the accumulation of PR proteins (White, R. F., Virology 99: 410-412 (1979). As a result, SA was considered as a possible endogenous signal molecule (Van Loon, L. C. et at., Neth. J. Plant. Path. 88: 237-256 (1982)). However, progress slowed through the 1980's and the involvement of PR proteins and salicylic acid in SAR was questioned.
With the advent of genetic engineering and recombinant DNA technology, the possibility of manipulating genetic material to improve the phenotype of plants has arisen. The present invention is based in part upon the discovery of the identity and role of genes involved in SAR which has allowed the application of modem molecular biological techniques for improved plant disease and plant pest resistance.